Method for selecting synchronous codes in a mobile communication synchronic system

ABSTRACT

The invention discloses a selecting method of synchronization codes for a mobile communication synchronization system. The method selects binary codes with zero correlation window as the synchronization codes for a mobile communication synchronization system, i.e. selects binary codes whose periodic autocorrelation and periodic cross-correlation have the zero correlation window, or whose aperiodic autocorrelation and aperiodic cross-correlation have the zero correlation window; and for former transmitting the synchronization codes uses cyclic and continuous mode and for later a guard period is added to each synchronization code for transmitting. A width of the zero correlation window is determined to be greater than time delay spread of a multipath signal. Using the method, the selected synchronization codes can implement more accurate synchronization.

FIELD OF THE TECHNOLOGY

The invention generally relates to wireless telecommunicationtechnology, specifically to a synchronization scheme with specificallyselected synchronization codes of the synchronization system in themobile communication technology.

BACKGROUND OF THE INVENTION

Synchronization is a key technique in a mobile communication system.Searching with a synchronization code, the maximum correlation peak canbe found through a correlation method; then the synchronization ofsubscriber signal is implemented or the most appropriate base stationfor the subscriber access is found. At present in a mobile communicationsystem, the selected synchronization codes are pseudo-random codes. Asthe autocorrelation and cross-correlation value of these pseudo-randomcodes at the neighborhood where time-delay equals to zero are not equalto zero, so there is mutual interference between these pseudo-randomcodes.

Code division multiple access (CDMA) is an interference limitedtechnology. When system synchronization is made with synchronizationcodes, the synchronization accuracy and veracity will be affected byinterference, which includes interference caused by the multipathpropagation of the synchronization signal itself and interference causedby the synchronization signals of other subscribers, etc.

For suppressing these interferences, a multi-user detection technique orother adaptive process methods are used at present, but they have highcomputation complexity and are difficult to realize. Therefore, with thepresent synchronization codes and without increasing the systemcomplexity, it is difficult to implement a low interference, highaccuracy and veracity of synchronization.

SUMMARY OF THE INVENTION

Objective of the invention is to design a synchronization scheme with aselecting method of the synchronization codes for a mobile communicationsystem, and the synchronization codes selected by this method willincrease synchronization accuracy without increasing the systemcomplexity.

For describing the technical scheme of this method, first the followingdefinitions will be introduced.

Definition 1:

Suppose a and b are two binary codes with equal length N,a=[a ₀ a ₁ . . . a _(N-1)] and b=[b ₀ b ₁ . . . b _(N-1)]; wherein a_(i) , b _(i)∈{−1,1}, i=0,1, . . . N-1;then

$\begin{matrix}{{r_{a}(\tau)} = {\frac{1}{N}{\sum\limits_{t = 0}^{N - 1}{a_{t}a_{{({t + \tau})}\mspace{11mu}\%\mspace{11mu} N}}}}} & (1) \\{{r_{a}^{(l)}(\tau)} = {\frac{1}{N}{\sum\limits_{t = 0}^{N - 1 - \tau}{a_{t}a_{t + \tau}}}}} & (2)\end{matrix}$

wherein τ=0,1 . . . N−1; and formulas (1) and (2) are periodicautocorrelation and aperiodic autocorrelation of the code a,respectively.

$\begin{matrix}{{r_{ab}(\tau)} = {\frac{1}{N}{\sum\limits_{t = 0}^{N - 1}{a_{t}b_{{({t + \tau})}\mspace{11mu}\%\mspace{11mu} N}}}}} & (3) \\{{r_{ab}^{(l)}(\tau)} = {\frac{1}{N}{\sum\limits_{t = 0}^{N - 1 - \tau}{a_{t}b_{t + \tau}}}}} & (4)\end{matrix}$

wherein τ=0,1 . . . N−1; % is a modulo operator and formulas (3) and (4)are periodic cross-correlation and aperiodic cross-correlation of thecodes a and b, respectively.

Definition 2:

Suppose spread spectrum codes m₁, m₂. . . m_(p) with length Nsimultaneously satisfy:

a. periodic autocorrelationr _(m) _(i) (τ)=0or aperiodic autocorrelation

r_(m_(i))^((l))(τ) = 0, i = 1, 2, …  , P, τ ∈ [−L, 0)⋃(0, L], L < N;

b. periodic cross-correlationr _(m) _(i) _(m) _(i) (τ)=0or aperiodic cross-correlation

r_(m_(i)m_(j))^((l))(τ) = 0, i, j = 1, 2, …  , P, i ≠ j, τ ∈ [−L, L], L < N,

then binary codes with the zero correlation characteristics are calledcodes with zero correlation window.

Technical scheme to implement objective of the invention is asfollowing:

A method of selecting synchronization codes for a mobile communicationsynchronization system, it is characterized that binary codes with zerocorrelation window characteristics are selected as the synchronizationcodes for a mobile communication synchronization system.

The binary codes with the zero correlation window characteristics arebinary codes whose periodic autocorrelation and periodiccross-correlation have the zero correlation window characteristics, orwhose aperiodic autocorrelation and aperiodic cross-correlation have thezero correlation window characteristics.

A width of the zero correlation window is determined to be larger than atime delay spread of a multipath signal.

For the synchronization codes whose periodic autocorrelation andperiodic cross-correlation have the zero correlation windowcharacteristics, the synchronization codes are transmitted by a cyclicand continuous transmitting mode.

The step of continuously transmitting the selected synchronization codesis that each selected synchronization code is repeatedly transmitteduntil a synchronization time slot with 96 chips is filled in.

For the synchronization codes whose aperiodic autocorrelation andaperiodic cross-correlation have the zero correlation window, a guardperiod is added to each synchronization code for transmitting.

The step of adding the guard period in the transmitted synchronizationcode is that, in the synchronization code including a 32 chips GP and a64 chips SYNC time slot, the 64 chips SYNC time slot is substituted by acode whose aperiodic autocorrelation and aperiodic cross-correlationhave the zero correlation window.

When making synchronization searching, the synchronization codes arecaptured according to a correlation method.

In the invention, the correlation window width of a selectedsynchronization code is determined according to a system requirement,which includes, but not limited, the followings: the multipath timedelay spread of a wireless signal in the mobile communication cell, chiprate in the mobile communication system etc.

In practice, with the selected codes of the invention, the capture ofthe synchronization codes accords to the ordinary correlation method.

The effect of the invention is as follow. Since the selectedsynchronization code has a better autocorrelation characteristics, themultipath signals caused by multipath propagation of the synchronizationcode itself does not produce interference; at the same time, since thecross-correlation between different synchronization codes also has zerocorrelation window, synchronization codes of other subscribers does notbring interference to the subscriber too. Therefore, without usingmultiple user detection technique or other adaptive process methods, thesynchronization codes selected by the method of the invention can havegood correlation detection to implement a more accurate synchronization.

When periodic autocorrelation and periodic cross-correlation of theselected synchronization codes have the zero correlation window, thesynchronization codes can be transmitted with a cyclic and continuoustransmitting mode. When aperiodic autocorrelation and aperiodiccross-correlation of the selected synchronization codes have the zerocorrelation characteristics, the synchronization codes can betransmitted with adding a guard period mode. Both of these modes canguarantee implementation of the effect mentioned above.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a structure diagram of a synchronization code selected withthe method of the invention.

EMBODIMENTS OF THE INVENTION

The invention will be described in more detail with reference the FIGUREand embodiment.

Refer to FIG. 1, the length of a synchronization code is 75 μs, theprotection time-slot GP is 32 chips and the synchronization time-slot(SYNC) is 64 chips. Since the synchronization code is an aperiodicautocorrelation and aperiodic cross-correlation synchronization codewith zero correlation characteristics, so a protection time-slot isadded.

In a time division-synchronous CDMA (TD-SCDMA) mobile communicationsystem, according the requirement of “Spreading and modulation” inreference document TS C104, the SYNC part of FIG. 1 is substituted witha code that is a aperiodic autocorrelation and aperiodiccross-correlation synchronization code with a zero correlation window,so the mutual interference of the synchronization codes is reduced.

A binary code with periodic autocorrelation and periodiccross-correlation with a zero correlation window characteristics, whichis proposed by document, “Spreading sequence sets with zero correlationzone”, Deng Xinmin and Fan Pingzhi, Electronics Letters, 2000, 36(11):993˜994, can also be used as synchronization codes, it is shown asformula (5) in the following:

$\begin{matrix}\begin{matrix}{a_{1} = \left( {--{--{+ {- {+ {--{+ {--{++{+ {- {+ {--{++{+ --}}}}}}}}}}}}}}} \right)} \\{a_{2} = \left( {--{+ {--{++{++{+ {- {++{+ {--{- {+ {--{- {+ {- +}}}}}}}}}}}}}}}} \right)} \\{a_{3} = \left( {+ {- {+ {--{- {+ {--{- {++{- {+ {--{--{- {++{- ++}}}}}}}}}}}}}}}} \right)} \\{a_{4} = \left( {--{++{+ {--{+ {- {++{++{+ {- {++{- {+ {- {++++}}}}}}}}}}}}}}} \right)}\end{matrix} & (5)\end{matrix}$

Wherein “−” represents “−1” and “+” represents “+1”. When transmittingthe synchronization code, a code such as a₁ is selected. Since there are96 chips in FIG. 1 synchronization code structure, so synchronizationcode a₁ must be transmitted repeatedly four times, i.e. a₁ a₁ a₁ a₁, tofill the synchronization time slot.

The code in formula (5) has a zero correlation window with three chipswidth, the periodic autocorrelation and periodic cross-correlation,which is computed by formulas (1) and (3), satisfy:r _(a) _(i) (τ)=0,i=1,2,3,4,τ=1,2,3 and r _(a) _(i) _(a) _(j)(τ)=0,i,j=1,2,3,4, i±j,τ=0,1,2,3

Suppose taking the arriving time of first path signal as reference forthe received multipath signal of a base station, the maximum relativetime delay of other paths is T_(max) and system chip period is Tc; thenwhen T_(max)<3Tc, the codes of the formula (5) can be used as the systemsynchronization codes, otherwise it is necessary to take a code with awider zero correlation window as the system synchronization codes.

When energy of multipath signals is relatively small, such as less than−20 dB, then affection to the system is smaller too. In this case, themaximum relative time delay of each of path T_(max) can take the maximumrelative time delay of multipath with enough signal energy.

1. A method for selecting synchronization codes for a mobilecommunication synchronization system, comprising: selecting binary codeshaving zero correlation window characteristics as synchronization codesfor the mobile communication synchronization system, wherein the binarycodes having the zero correlation window characteristics are binarycodes whose periodic autocorrelation and periodic cross-correlation havethe zero correlation window characteristics, or binary codes whoseaperiodic autocorrelation and aperiodic cross-correlation have the zerocorrelation window characteristics.
 2. The method according to claim 1,wherein a width of the zero correlation window characteristic isdetermined to be larger than a time delay spread of a multipath signal.3. The method according to claim 1, further comprising, for thesynchronization codes whose periodic autocorrelation and periodiccross-correlation have the zero correlation window characteristics,transmitting the synchronization codes by a cyclic and continuoustransmitting mode.
 4. The method according to claim 3, whereintransmitting the synchronization codes by the cyclic and continuoustransmitting mode comprises, repeatedly transmitting eachsynchronization code until a synchronization time slot with 96 chips isfilled in.
 5. The method according to claim 1, further comprising, forthe synchronization codes whose aperiodic autocorrelation and aperiodiccross-correlation have the zero correlation window characteristics,transmitting the synchronization codes by adding a guard period to eachsynchronization code.
 6. The method according to claim 5, wherein thetransmitting the synchronization codes by adding a guard period to eachsynchronization code comprising, for the synchronization code whichincludes a 32 chips guard period and a 64 chips sync time slot,substituting the 64 chips sync time slot by the synchronization codewhose aperiodic autocorrelation and aperiodic cross-correlation have thezero correlation window.
 7. The method according to claim 1, furthercomprising, capturing the synchronization codes according to acorrelation method in a synchronization searching procedure.